The present invention relates to a mechanical pump or more specifically to a pump structure suitable for applications immersed in liquid metal.
Generally, the mechanical pump handling a liquid metal such as liquid sodium is a vertical type centrifugal volute pump with its lower part immersed in the liquid metal.
The pump has a structure in which an impeller is mounted to the lower end of the rotating shaft and a hydrostatic bearing which supports the rotating shaft with the delivery pressure of the sodium is provided above the impeller.
There are two types of mechanical pumps that handle liquid metals: one is a dual casing type as introduced in the U.S. Pat. No. 3,467,015; and the other is a single casing type as disclosed in the U.S. Pat. No. 4,219,385.
In the former type, the inner casing suspends an internal structure including the hydrostatic bearing, suction pipe, diffuser and delivery pipe and the outer casing accommodates the entire internal structure including the rotating shaft and the impeller. The internal structure as a whole can readily be removed out of the casing.
In the latter type, a part of the lower portion of the pump is supported in a vessel independently of an upper structure of the pump. The upper structure includes a large enclosed cylindrical casing, a bearing secured to the casing and a pump shaft supported by the bearing. In the single casing type, the casing which directly supports the bearing cannot be eliminated and should be made strong enough to support the bearing.
The conventional pump has either a single casing or dual casings. Several problems have become evident with these conventional structures.
A few typical problems are described in the following. In the conventional pumps with dual casings, one inside and the other outside, as described in the U.S. Pat. No. 3,467,015, a natural convection causes upward and downward flows of a cover gas (usually a mixture of argon gas and sodium vapor) loaded in an annular gap between the inner casing and the outer casing, producing a temperature distribution along the circumferential direction of the casing. This in turn deforms the casings, giving rise to the possibility that the rotating shaft will contact a member of the pump and be galled, halting the shaft rotation.
The pumps with a structure such as described in the U.S. Pat. No. 4,219,385 have no such problem. But they still have problems common to those pumps with the conventional casings--a heat treatment required to remove strains caused by welding of the casing and an exposure of maintenance workers to radiations. The solution to the former problem requires a large facility for heat treatment. But even with the facility made available a problem still remains that it is difficult to make the large cylindrical casing homogeneous in quality by heat treatment. Moreover, cleaning or decontamination of the internal structure, in which radioactive corrosion products easily collect, cannot effectively be performed because of the presence of the casing. The casing also contributes to increasing the area to which the radioactive corrosion products adhere, and makes disassembly job difficult.